1. Field of the Invention
The present invention relates to a modem apparatus for realizing data communications through a communication network such as a dial-up line, a dedicated line, etc. Also, the present invention relates to a data communication method using such a modem apparatus.
2. Description of the Related Art
In recent years, in the field of data communications, V.34 modem (33.6 kbps) has been widely used in accordance with Recommendation of ITU-T. Particularly, in the field of the facsimile apparatus, data communications have been widely carried out along communication standard of T. 30ANEXF(so-called SUPER G3) using V.34 modem. In the case of using V.34 modem in the facsimile apparatus, specified pre-protocols must be carried out until an image signal is transmitted after a line connection between the facsimile apparatuses on sender and receiver sides is established. In the pre-protocols, there are included a protocol for selecting a data communicable modulation mode from modulation modes such as V.34 half-duplex, V.34 full-duplex, and V.17 half-duplex, and a protocol for selecting a training parameter based on a line probing tone transmitted from the sender side. Further, there are included a protocol for transmitting a training signal from the sender side based on the selected training parameter, and a protocol for checking a receiving quality on the receiver side based on the received training signal. Furthermore, there is included a protocol for selecting a suitable modem parameter based on the checked receiving quality.
For example, in a case where image data is transmitted at the maximum communication rate of 33.6 kbps of V.34 modem, image data can be transmitted in approximately three seconds per sheet of paper of size A4. In such a case, however, extra time of about seven seconds is needed to carry out the aforementioned pre-protocols, and much communication time is wasted as compared with image transmission time.
In order to solve the above problem, inventors of the present invention have proposed a technique of reducing time required in the pre-protocols as possible. According to this technique, the modem parameter best suited to perform communications with a specific destination is stored at an actual image transmitting time. Then, the modem parameter stored is used in receiving and transmitting the image from/to the specific destination at the next time so as to shorten the pre-protocols.
However, in the above-proposed technique, communication errors may occur in some cases at the time hen the pre-protocols are shortened and the modem parameter is transmitted to the receiver side. This may be caused by the following reason. More specifically, the modulated transmitting signal is easily influenced by noise (impulse), and it is thereby difficult to detect the transmitting signal on the receiver side.